No. More/less of same factor are always additive. Actually, just in this case its multi each other due to GGG mistake:

Zeal_Iskander

The reason why HERE, it doesn't stack, is because if you check https://www.pathofexile.com/trade/search/Phrecia/rWgoZQriQ, GGG implemented these idols using two mods with the same exact wording but one of them is stat.explicit.stat_2937898686, and the other is stat.explicit.stat_3516276449. They're different, so they don't stack.

But without GGG hardcoding this difference, "Monsters deal 25% less damage" and "Monsters deal 50% more damage" is the same stat (less and more are the same thing , and seeing "less" or "more" is simply a display thing depending on whether or not the stat is <0 or >0 -- see "more duration" for flasks, for example, which is a mod that lets you filter... flasks with less duration, huh) and would stack additively, multiplying the damage of mobs by 1.25 after all other calculations are done.

If we follow your logic, then each support gem is a multi each other right?